A number of offset printing plate precursors for directly producing printing plates have hitherto been proposed, and some of them have already been put into practical use. Widely employed among them is a system in which a photoreceptor comprising a conductive support having provided thereon a photoconductive layer mainly comprising photoconductive particles, e.g., zinc oxide, and a resin binder is subjected to ordinary electrophotographic processing to form a highly lipophilic toner image thereon, and the surface of the photoreceptor is then treated with an oildesensitizing solution called an etching solution to selectively render non-image areas hydrophilic to obtain an offset printing plate.
Requirements of offset printing plate precursors for obtaining satisfactory prints are such that: an original should be reproduced faithfully on the photoreceptor; that the surface of a photoreceptor has affinity with an oildesensitizing solution so as to render non-image areas sufficiently hydrophilic and, at the same time, has water resistance; and that a photoconductive layer having formed thereon an image is not attenuated during printing and is sufficiently receptive to dampening water so that the non-image areas retain the hydrophilic properties enough to exhibit freedom from stains even on printing a large number of prints.
For a particular use as an offset printing plate precursor, formation of background stains due to insufficient oil-desensitivity presents a serious problem. In order to solve this problem, various resins as binders for zinc oxide have been proposed, for example, as disclosed in Japanese Patent Publication No. 31011/75, Japanese Patent Application (OPI) Nos. 54027/78, 20735/79. 202544/82, and 68046/83 (the term "OPI" as used herein means an "unexamined published Japanese patent application"). Nevertheless, actual evaluations of these resins proposed for improving oil-desensitivity revealed that none of the lithographic printing plate precursors containing them was satisfactory in terms of stain resistance and printing durability.
On the other hand, a photoconductive layer comprising particles of an organic photoconductive compound and a binder resin can be formed on a grained aluminum base. Printing plate precursors of this type can be produced by forming a toner image on a photosensitive layer through the known electrophotographic processing as described above and removing the non-image areas by eluting with a processing solution, whereby the aluminum base corresponding to the non-image areas is exposed to form hydrophilic areas. The photosensitive layer using the organic photoconductive compound comprises, for example, an oxadiazole compound or oxazole compound and an alkali-soluble binder resin, e.g., a styrene-maleic anhydride copolymer, etc., as disclosed in Japanese Patent Publication Nos. 17162/62 and 39405/71 and Japanese Patent Application (OPI) Nos. 2437/77 and 107246/81; or a phthalocyanine pigment or azo pigment and an alkali-soluble phenolic resin as disclosed in Japanese Patent Application (OPI) Nos. 105254/80, 16125/70, 150953/83, and 162961/83.
However, since the above-mentioned plate making process involves removal of the photosensitive layer corresponding to the non-image areas by dissolution, it requires a large-sized apparatus and is too time-consuming to achieve a reasonable production rate. Besides, use of an organic solvent as processing solution, such as ethylene glycol, glycerin, methanol, ethanol, etc., encounter problems of cost safety, environmental pollution, working hygiene, and the like.
It has been proposed to provide a specific resin layer capable of being rendered hydrophilic on a usual electrophotographic photoreceptor as described in Japanese Patent Publication No. 5606/70. According to this system, a surface layer comprising a vinyl ether-maleic anhydride copolymer and a hydrophobic resin compatible with the copolymer is provided on the photosensitive layer. After toner image formation, the non-image areas of the surface layer can be rendered hydrophilic by treating with an alkali to hydrolytically open the anhydride ring.
However, since the vinyl ether-maleic anhydride copolymer used in the surface layer becomes water-soluble upon ring-opening, the surface layer is seriously inferior in water resistance even when the surface layer is composed of the vinyl ether-maleic anhydride copolymer in combination with other compatible hydrophilic resins. Therefore, printing durability of the resulting printing plate is only about 500 to 600 prints at most.
It has also been proposed to form a surface layer capable of being rendered hydrophilic, which comprises silylated polyvinyl alcohol as a main component and a crosslinking agent as disclosed in Japanese Patent Application (OPI) Nos. 90343/85, 159756/85, and 217292/86. According to this proposal, after toner image formation, the surface layer on the non-image area can be rendered hydrophilic by hydrolysis of the silylated polyvinyl alcohol. Further, in order to retain film strength after becoming hydrophilic, the degree of silylation of polyvinyl alcohol is so controlled, and the remaining hydroxyl groups are crosslinked by the crosslinking agent.
The above publications describe that the provision of such a surface layer would prevent background stains of prints and increase printing durability. However, it has been determined by evaluation under conditions of actual use that the resulting printing plate still does not satisfy the stain resistance requirement. The above method has the further disadvantages as follows. In nature of high-molecular reaction, it is difficult to stably produce the silylated polyvinyl alcohol having a desired degree of silylation and to quantitatively react the functional groups in the polymer. Quantitative determination of the reaction degree requires an expensive and highly precise analytical instrument. The method involves incorporation of by-products or impurities resulting from the high-molecular reaction and the like into the product, which is unfavorable for stabilization of performance, such as electrophotographic characteristics. The residual hydroxyl groups cannot be completely crosslinked even by using a crosslinking agent, and the surface layer is rendered conductive by the remaining hydroxyl groups and, therefore, becomes liable to cause charge leakage. As a result, the surface layer can hardly hold insulation quality equal to that of a photosensitive layer, thus making it difficult to avoid adverse influences on the surface layer, such as reduction in exposure sensitivity. In addition, the chemical structure of the hydrophilic polymer has limitations which make it difficult to improve charging properties and quality of a reproduced image (e.g., dot reproducibility and resolving power of image areas, resistance to background fog of non-image areas, etc.) without adversely influencing other qualities of an electrophotographic photoreceptor.
Furthermore, the above method entails cost since achievement of sufficient hydrophilic properties after hydrolysis requires a relatively large quantity of silylated polyvinyl alcohol. Besides, considering the whole balance of performance, such as electrophotographic characteristics, film-forming properties, etc., there is only a narrow choice of material composition.